def prost(x):
    k = 0
    for d in range(1,x+1):
      if x%d ==0:
          k+=1
    if k==2:
           return True
    else:
        return False
maxx = 0
num= int(input())
if num>= 1000 and num <= 9999:
  n1 = str(num)
  for cifra in range(len(n1)):
      if prost(int(n1[cifra])) == True:
         maxx = max(maxx,int(int(n1[cifra])))
  for chislo in range(len(n1)-1):
      if prost(int(n1[chislo]+n1[chislo + 1])) == True:
         maxx = max(maxx,int(int(n1[chislo] +n1[chislo+1])))
  for chislo in range(len(n1)-2):
      if prost(int(n1[chislo]+n1[chislo + 1] +n1[chislo + 2])) == True:
         maxx = max(maxx,int(int(n1[chislo] +n1[chislo+1] +n1[chislo+2])))
  for chislo in range(len(n1)-3):
      if prost(int(n1[chislo]+n1[chislo + 1]+n1[chislo + 2]+n1[chislo + 3])) == True:
         maxx = max(maxx,int(int(n1[chislo] +n1[chislo+1] +n1[chislo+2] +n1[chislo+3])))
  print(maxx)
else:
    print(0)

